1. Field of the Invention
This invention concerns compatible blends (polyblends) consisting of polyoxymethylene and polyalkyl acrylates.
2. Discussion of the Background
Different polymeric species are generally considered not to be compatible with one another, i.e., different polymeric species do not generally develop any homogeneous phase down to small proportions of one component in a second component that would be characterized by complete miscibility of the components. Certain exceptions to this rule have brought about increasing interest, especially among those concerned with the theoretical interpretation of the phenomena.
Completely compatible blends of polymers show complete solubility (miscibility) in all blend ratios. The glass transition temperature Tg or the so-called "optical method" (clarity of a film cast from a homogeneous solution of the polymer blend) have frequently been used to prove miscibility. (See Brandrup-Immergut, Polymer Handbook, 2nd Ed., III-211-213; Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Ed. Vol. 18, 443-478, J. Wiley & Sons 1982).
Thus, for example, a blend of polyethylene oxide with polyacrylic acid shows a higher Tg with a low polyethylene oxide content than either of the two components themselves. (See K. L. Smith, A. E. Winslow & D. E. Petersen, Ind. Eng. Chem. 51, 1361 (1959)).
Polyoxymethylene (POM), because of its desirable mechanical properties (hardness, rigidity, toughness, down to low temperatures) and its resistance to solvents, has found a firm position among engineering materials. Thus, about 190,000 tons of POM was already consumed worldwide in 1983. (See R. Vieweg, M. Reiher, H. Scheurlen, Ed., Plastics Manual Vol. 11, Carl Hanser Verlag Munich 1971; G. E. Haddeland in Process Economics Program Report No. 69, Acetal Resins, Stanford Research Institute, Menlo Park, USA (1971); Kirk-Othmer, Encyclopedia of Chemical Technology 3rd Ed. Vol 1, pp. 112-123, J. Wiley 1978; Winnacker-Kuchler, Chemische Technologie Vol. 6, Org. Technologie II, 4th Edition, Carl Hanser, Munich 1982).
The literature provides a number of examples of mixtures of polyoxymethylene with other plastics, apparently in the pursuit of various technological objectives, but without true compatibility of the components being intended or even achieved. Thus, DE-A 27 09 037 describes a coating paste for missile propellant charges that was obtained from a solution of POM, polymethyl methacrylate (PMMA), and paraformaldehyde in toluene. (See Chem. Abstr. 90:206 733h). The ability of the POM to form crystalline fibers has been utilized variously to produce fiber-reinforced plastics, including those based on PMMA (See Chem. Abstr. 83:148376m; Chem. Abstr. 87:85985u).
Attempts have been made to improve the impact strength of POM by mixing with elastomers, for example acrylonitrile-grafted ethylene-propylene rubber (Chem. Abstr. 99:176894c) or MMA-grafted polybutadiene (See DE-A 3 441 547), or butadiene-MMA block copolymers (DE-A 24 20 300) or butadiene-styrene or acrylonitrile-butadiene-styrene graft copolymers (DE-A 19 31 392). Other impact strength modifications are acrylonitrile/styrene-grafted or styrene-grafted polyoxymethylene (DE-A 26 59 357). The preparation of high-impact strength POM by polymerization of trioxane in the presence of an elastomer such as an ethylene-propylene-2-hydroxyethyl methacrylate copolymer is recommended in JP-A 60/108413 (Chem. Abstr. 104: 6621r) (See; also Chem. Abstr. 103: 19692v). EP-A 115 373 recommends the addition of a multiple-phase crosslinked copolymer to POM mixtures with an alkyl C.sub.10-30 fatty acid C.sub.2-7 ester and polymers such as caprolactam-caprolactone copolymers or polybutyl methacrylate, which are compatible with the fatty acid ester and are inert to POM.
The special tendency of POM toward crystallization, which in the last analysis also amounts to the high mechanical strength and the good resistance to solvents, as well as the fact that the material tends to decompose by splitting off formaldehyde (ceiling temperature: 127.degree. C.) even 50.degree. C. above the crystallite melting point (m.p.: 175.degree.-184.degree. C.), of course permits only a very narrow processing range, so that up to this time, approximately 90% of the overall consumption of POM is in the field of injection molding, since subsequent forming of panels, for example, by deep-drawing or the like is possible only with difficulty because of the very narrow processing range of this material. On the whole, the homopolymers and copolymers of the POM type are considered to be unstable to heat and oxygen, so that they can be processed only after the addition of suitable stabilizers. (See H. Batzer Ed., Polymere Werkstoffe, Volume III, Technologie 2, pp. 144-148 ff; Volume II, pp. 375-376, Georg Thieme Verlag, Stuttgart, New York 1984; U.S. Pat. No. 3,081,280). Since the decomposition of polyoxymethylene proceeds from the chain ends containing hydroxy groups, practically all commercial grades of POM are stabilized by terminal esterification or etherification. Under the conditions of use in practice, of course, this stabilization is inadequate since, for example, under the action of the formic acid (formed from the split-off formaldehyde), polymeric fragments are again formed with thermolabilizing terminal hydroxy groups. For this reason, aldehyde-binding and acid-binding additives such as urea-melamine derivatives, hydrazine derivatives, and polymer-compatible copolyamides (such as a PA-6/PA-66 copolyamide) are usually added, together with antioxidants (See H. Batzer, loc.cit. Vol. II, pp. 375 376). Therefore, the problems in processing POM are appropriately characterized by the statement: "that the POM consumption of a country depends on its degree of industrialization" (H-D Sabel, Kunststoffe 70, 641 (1980)). It has therefore been necessary to direct the efforts of technology toward improving POM by modifying its processability and manageability in the form of its homopolymers and copolymers, if possible without negatively affecting its beneficial characteristics.